1. Field of the Invention
The present invention relates to a printer in which a plurality of printing units are disposed about a platen roller. A printing medium is delivered by the roller to the printing units sequentially as the roller rotates so that a desired image can be printed on the medium by each printing unit. Each printing unit sequentially prints a specific ink layer color of the desired image in an overlapping manner.
2. Description of the Related Art
In a printer employing a plurality of ink ribbons of which different color inks are applied, the ink of each ink ribbon is transferred to a printing medium in an overlapping manner to obtain a desired image.
Known printers include a platen roller rotated by a stepping motor and printing units with a plurality of different colors (yellow (Y), magenta (M), cyan (C), and black (K)) arranged about the platen roller at a predetermined interval along a delivery path of the printing medium.
Each printing unit individually includes an ink ribbon, a printing head, and a platen roller. A first printing unit, a second printing unit, a third printing unit, and a fourth printing unit are arranged from an upstream side to a downstream side.
Specifically, each printing unit is disposed tangentially around the platen roller and opposite the printing medium, with the ink ribbon between the platen roller and the printing head. The printing head and the platen roller can be pressed together with the ink ribbon and the printing medium therebetween.
Additionally, the printing medium is delivered from a roll from an upstream side to a downstream side along the delivery path defined by rotation of the platen roller.
In known printers, in order to print a desired image on the printing medium, electrification or activation of a thermal head of the first printing unit is performed. For example, using thermal-transfer of an ink layer of the ink ribbon of yellow (Y), a first image is printed on the printing medium. Sequentially, the area of the printing medium in which the first image is printed is delivered to a second printing unit as the platen roller rotates, where the second printing unit is arranged downstream from the first printing unit. A second image, for example, using an ink layer of an ink ribbon of magenta (M) is applied over the first image by a thermal head of the second printing unit.
Next, the area in which the first image and the second images are printed is delivered to a third printing unit, located further downstream along the delivery path. A third image, for example, using an ink layer of an ink ribbon of cyan (C), is applied over first image and the second image by a thermal head of the third printing unit.
Sequentially, the area in which the first image to the third images are printed is delivered to a fourth printing unit arranged further downstream along the delivery path. A fourth image, for example, using an ink layer of an ink ribbon of black (K), is applied over the first image to the third images by a thermal head of the fourth printing unit. Accordingly, the desired color image is printed on the printing medium (for example, see JP-A-2003-291388).
The printing medium is fed from the paper roll arranged at the uppermost side of the delivery path and is fed to a feed roller and is taken up on an outer surface of the platen roller. The printing medium is provided to each thermal head sequentially from the first printing unit to the fourth printing unit by the rotation of the platen roller such that each ink layer of the ink ribbon is transferred to the printing medium. Once the image is printed, the printing medium is fed to a discharge roller and then is discharged out of the printer, and is cut at a predetermined length by a cutting device.
In such a printing medium, a new roll of printing medium is mounted in the printer and delivered along the outer surface of the platen roller in order to start an initial printing job. In this case, when the leading edge of the printing medium is initially delivered to first printing unit located on the upstream side of the delivery path, the printing medium is not fully wound about the platen roller. Thus, when the first printing unit prints first image on the printing medium, friction between the platen roller and the printing medium is reduced because the printing medium is not fully wrapped about the platen roller, and slippage of the printing medium occurs.
When the second image is printed by the second printing unit, the printing medium is wound about half way about the platen roller. Accordingly, the friction between the platen roller and the printing medium is less than normal, and slippage occurs during printing, and thus the length of the second image is shorter than a desired length. However, the second image is longer than the first image, which experienced even greater slippage.
Additionally, when the third image is printed by the third printing unit, the friction between the platen roller and the printing medium is still less than normal. Accordingly, the slippage still occurs, and thus the length of the third image is shorter than a desired length. Again, however, the third image is longer than the second image.
When the fourth image is printed by the fourth printing unit, the printing medium is fully wound on the platen roller to the same amount that occurs during normal (continuous) printing. Because the printing medium is fully wound about the platen roller, thus insuring sufficient friction between the platen roller and the printing medium, it is difficult for slippage to occur. As a result, the fourth image can be printed at an approximately desired length.
There is a difference in the amount of slippage, and hence a difference in the length of the printing medium properly fed to the printing units between the initial printing by the first printing unit and the printing by the fourth printing unit, that is, when the printing medium is fully wound on the outer surface of the platen roller. This difference is proportional to the area of contact of the printing medium relative to the platen roller. Accordingly, color smears from the first image to the fourth image exist in the initial printed result.
When the printing medium is fully wound on the outer surface of the printing medium by a proper grip of the platen roller, slippage of the printing medium relative to the platen roller does not occur.
As described above, color smear results from the proportional difference in the friction between the printing medium and the platen roller as the printing medium is initially fed onto the platen roller from the roll, to the time the leading edge of the printing medium is wrapped fully around the platen roller.
One way to guarantee the required amount of friction is by not printing until the leading edge of the printing medium approaches the discharging roller arranged at the most downstream portion of the delivery path. This can be done at the time of initial printing by winding the printing medium on the platen roller until it is fully wound about the platen roller. However, in this case, the initial portion of the printing medium is wasted, which leads to unnecessary consumption of the printing medium, which is uneconomical.